In addition to our recent report on the potent anti-varicella-zoster virus (VZV) activity of some unusual bicyclic furopyrimidine nucleosides bearing long alkyl side chains, we herein report the further significant enhancement of the antiviral potency by inclusion of a phenyl group in the side chain of these compounds. The target structures were prepared by the Pd-catalyzed coupling of a series of para-substituted arylacetylenes with 5-iodo-2'-deoxyuridine, to give intermediate 5-alkynyl nucleosides which were cyclized in the presence of Cu to give the desired bicyclic systems. The compounds display extraordinary potency and selectivity for VZV; the most active are ca. 10 000 times more potent than the reference compound acyclovir and ca. 100 times more potent than the alkyl analogues earlier reported by us. The current compounds show little cytotoxicity, leading to selectivity index values >/= 1 000 000. From a range of DNA and RNA viruses tested, only VZV was inhibited by these compounds indicating their extreme selectivity for this target virus. The novelty of the molecules, coupled with their extreme potency and selectivity, their desirable physicochemical properties, and their relative ease of synthesis, makes them of considerable interest for potential drug development for VZV infections.

Bicyclic furano pyrimidine nucleosides have been found to be highly potent and selective inhibitors of varicella zoster virus (VZV). They are inactive against herpes simplex virus and have been known for several decades as (unwanted) synthetic by-products in the Pd-catalysed coupling of acetylenes to 5-iodo nucleosides. These fluorescent bicyclic nucleosides are now established as a new family of potent antivirals. They are unusual in that they exhibit complete specificity for VZV and require an alkyl (or alkylaryl) side-chain for biological activity. The latter requirement confers extremely high lipophilicities on these compounds, unknown amongst chemotherapeutic nucleosides, which may be of considerable importance in formulation, dosing and tissue distribution. The most potent compounds reported are p-alkylaryl compounds, with EC50 values below 1 nM versus VZV and selectivity index values of around 1,000,000. Here, we review the discovery, synthesis, characterization, antiviral profile, SAR, mechanism of action and development prospects for this new family of antivirals.

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The susceptibility of the bicyclic nucleoside analogs (BCNAs), highly potent and selective inhibitors of varicella-zoster virus (VZV), to the enzymes involved in nucleoside/nucleobase catabolism has been investigated in comparison with the established anti-VZV agent (E)-5-(2-bromovinyl)-2Ј-deoxyuridine [BVDU; brivudine (Zostex)]. Whereas human and bacterial thymidine phosphorylases (TPases) efficiently converted BVDU to its antivirally inactive free base (E)-5-(2-bromovinyl)uracil (BVU), BCNAs showed no evidence of conversion to the free base in the presence of these enzymes. The lack of substrate affinity of TPase for the BCNAs could be rationalized by computer-assisted molecular modeling of the BCNAs in the TPase active site. Moreover, in contrast with BVU, which is a potent and selective inhibitor of dihydropyrimidine dehydrogenase (DPD) (50% inhibitory concentration; 10 M in the presence of a 25 M concentration of the natural substrate thymine), the free at a concentration of 250 M. Consequently, whereas BVU caused a dramatic rise of FU levels in FU-treated mice, the BCNAs did not affect FU levels in such mice. From our data it is evident that BCNAs represent highly stable anti-VZV compounds that are not susceptible to breakdown by nucleoside/ nucleobase catabolic enzymes and are not expected to interfere with cellular catabolic processes such as those involved in FU catabolism.